Why does the dope sheet use (inconsistent) white for unselected / deselected?
It is set that way as they are the colours that were chosen.

You can change both selected and deselected colours in the theme settings if you feel other colours work better. We can't set a transparency with the keyframes but if you set it the same as the background colour then only the border is seen to make it appear hollow. The keyframe border below the keyframe colours is the outline of the marker.